1. Field of the Invention
The invention relates to an apparatus for detecting the longitudinal edges of a rod-shaped object moved in its longitudinal direction, in particular a tobacco or filter rod.
2. Description of the Prior Art
Although similar problems are also encountered with other rod-shaped objects, for example pencils, or cables, wires, etc., hereinafter only the corresponding problems in the production of smokable articles, that is cigarettes, in particular filter cigarettes, will be discussed.
To ensure a constant high quality in the production of (filter) cigarettes the diameter on the one hand and the roundness of the tobacco rod and/or filter rod on the other hand should be constantly monitored. The output signals of the associated preferably automatically operating diameter and roundness measuring systems could be used on deviations from predetermined desired values to make the necessary corrections in the production processes via a closed-loop control system.
Because of the high rod velocity, generally of the order of some meters per second, for this purpose only a contactless measuring technique can be used. German OS No. 3,302,951 discloses a measuring method operating with ultrasonic waves; pneumatic methods and optical scanning methods using laser beams are already available on the market.
In the pneumatic method the material rod to be measured runs through a nozzle whose diameter is only somewhat greater than the maximum rod diameter. An air flow is blown through the annular gap remaining between the nozzle and rod and the flow resistance of the gap determined, it then being possible to calculate the cross-sectional area of the rod from the flow resistance. This measuring method does not enable any information to be obtained on the roundness of the rod and consequently the diameter of the rod can only be determined if it is assumed that a perfect cylindrical body is involved.
A further difficulty in the practical use of this method resides in the air permeability of the cigarette or filter paper which moreover can change in the course of time, resulting in corresponding measurement errors. A higher air permeability increases namely the air throughput, giving the appearance of a smaller diameter.
Optical scanning methods operating with laser beams are for example known from U.S. Pat. Nos. 3,856,411, 3,765,774, 3,905,705, 3,975,102 and the prospectus of the Zygo company "Beruhungsfreies Laser-Me.beta.system" ("Contactless laser measuring system").
An apparatus also operating with laser beams for detecting the longitudinal edges of a rod-like object of the aforementioned type moving in its longitudinal direction is also known from U.S. Pat. No. 3,853,406 and comprises a light source for generating a light beam, at least one beam divider dividing the beam coming from the light source into at least two light beams sweeping over the edge regions of the object, an optical deflection system for sweeping the two light beams over the object, at least two photodetectors disposed behind the object in the beam direction and an evaluating means for determining the position of the longitudinal edges from the instants of the bright/dark transitions at the two photodetectors on the one hand and the instantaneous position of the light beams on the other. The passage of the two beams at the longitudinal edges of the object is determined via the first and second derivative of the output signals of the two photodetectors in order to find exactly the instant at which the centre of each laser beam coincides with an edge of the object.
Thus, in principle the desired diameter is determined from the time interval of the bright/dark transitions compared with the travelling speed of the beam. Since this method is independent of the air permeability of the cigarette or filter paper, fundamentally very good measuring accuracy of about 1 .mu.m can be achieved.
However, the following problem then arises: Due to transverse rod oscillations or the rapid longitudinal movement of a rod which is not perfectly cylindrical transverse velocities of the order of magnitude of 10 to 100 mm/s can occur and lead to measurement errors of the order of magnitude of 10 to 100 .mu.m in the diameter. Although by taking an average of a large number of measured values a mean value, for example for 100 cigarettes, can be calculated with a substantially smaller error, the answer to the important question in practical assessment of quality as regards the change of diameter from one cigarette to the next or from one end of a filter cigarette to the other cannot be obtained because of the errors originating from the transverse movements.